The present invention relates to an improvement in a wire electrodischarge machine, particularly as regards the machining accuracy. The present invention is directed to a wire electrodischarge machine comprising upper and lower nozzles disposed above and below a workpiece and guiding an electrode wire, wherein a cross table, on which the workpiece is mounted, is moved in X and Y directions for relative movement of the workpiece and the electrode wire, while one of the upper and lower nozzles is moved in U and V directions in the horizontal plane for inclining the electrode wire relative to the workpiece for the purpose of taper machining.
FIG. 1 is a diagram showing this type of wire electrodischarge machine in the prior art, shown for example in Japanese Patent Application Kokai Publication No. 297019/1986. As illustrated, it comprises a machine main body 1 with a bed 2, a column 3, a cross table 4 driven on the bed 2 in directions of the horizontal plane, a table 5 which is fixed on the cross table 4 and on which a workpiece 6 is fixed, an arm 7 extending from the lower part of the column 3 toward the lower part of the table, a lower nozzle 8 mounted to the tip of the arm 7, a wire guide 9 in the form of a diamond die having a perforation on axis of the lower nozzle 8, an upper nozzle 10 which is disposed opposite to the lower nozzle 8 with respect to the workpiece 6 and supported to the nozzle drive device 12, which is mounted to the tip of the horizontal part 3b of the upper part of the column 3, and a wire guide 11 in the form a diamond die having a perforation on the axis of the upper nozzle 10.
A drive motor 13 is provided to drive the cross table 4 in the X direction. A similar motor, not shown, is provided to drive the cross table 4 in the Y direction. A drive motor 14 is provided for driving the upper nozzle 10 in the U direction, which is one of the directions in the horizontal plane in which the upper nozzle is driven to incline the wire in the workpiece for taper machining. Another drive motor, not shown, is provided for driving the upper nozzle 10 in the V direction, which is the other of the directions in the horizontal plane in which the upper nozzle is driven to incline the wire in the workpiece for taper machining.
An electrode wire 16 is wound on a wire bobbin 15 and unwound from it when the machining is conducted. A brake device 17 is provided for applying necessary tension on the electrode wire 16 to prevent the wire from becoming loose. The electrode wire 16 is passed over pulleys 115 and 118. A wire winding motor 18 is disposed on the downstream end of the electrode wire supply path. A wire collection box 19 recovers the used electrode wire. An NC (numerical control) device 20 is connected to the motors 13 and 14, as well as the motors, not shown, to control the movement of the cross table in the X and Y directions, and the movement of the upper nozzle 10 in the U and V directions.
A dielectric fluid 21 is supplied from a dielectric fluid supply device 22.
As will be seen from the enlarged view of FIG. 2, the nozzle drive device 12 comprises a vertical shaft 23 supported by the column 3 such that it can move up and down and driven by a drive mechanism, not shown, a guide member 24 fixed to the lower end of this vertical shaft 23 and having a dovetail groove 24a on its lower surface, a V slider 25 having, on its upper surface, a dovetail 25a slidably engaging with the dovetail groove 24a of the guide member 24 to be guided in the V direction (the direction normal to the surface of the drawing), and having a dovetail groove 25b formed on its lower surface, and extending perpendicularly to the dovetail 25a, and a U slider 26 which is in the form of a crank extending forward and downward, which has, on the upper surface of its root part, a dovetail 26a slidably engaging with the dovetail groove 25b of the V slider 25 to be guided in the U direction (lateral direction on the drawing), and having the upper nozzle 10 mounted to the lower surface of its tip part. When the motor 14 and another motor, not shown, rotate, the upper nozzle 10 deviates in the U and V directions from the reference position at which taper machining is not conducted.
The operation of the prior-art apparatus having the above configuration will now be described. The electrode wire 16 drawn from the wire bobbin 15 is supplied to the machining section via the brake device 17, and the pulley 115. After passing through the workpiece for electrodischarge machining, the wire is passed over the pulley 118, is wound by the motor 18, and is collected in the wire collection box 19. The wire 16 is supported and positioned by the wire guide 11 of the upper nozzle 10 and the wire guide 9 of the lower nozzle 8. the machining energy is supplied from a machining power supply, not shown, through a feeder, not shown, to the wire, and hence to the space between the wire 16 and the workpiece 6.
During electrodischarge machining, in accordance with the commands given from the NC device 20 to the drive motor 13, and another motor, not shown, the drive motor 13, and another motor, not shown, operate and the cross table 4 moves in the X and Y directions. As a result, the workpiece 6 mounted to the table 5 on the cross table moves relative to the wire 16, and is machined into a desired shape.
When a taper machining is conducted on the workpiece 6, the motor 14 is driven in accordance with the commands from the NC device 20 to move the U slider 26 in the U direction, and/or a motor, not shown, is driven in accordance with the commands from the NC device 20 to move the V slider 25 in the V direction, to incline the wire 16.
During machining, a dielectric fluid 21 is supplied at a high pressure from the dielectric fluid supply device 22 to the machining section via the upper and lower nozzles 10 and 8. The gaps between the upper and lower nozzles and the workpiece 6 are normally controlled to be within 0.01 to 0.02 mm.
In the machining by use of wire in a wire electrodischarge machine, it is necessary to conduct the discharge in a liquid, because the wire breaks if discharge occurs in the air and machining becomes disabled. Accordingly, as the higher machining speed is demanded in recent years, the tendency is to increase the dielectric fluid pressure in order to secure the adequate supply of dielectric fluid to the wire during machining. However, when the liquid pressure of the dielectric fluid 21 supplied to the machining section during machining is increased, the lower nozzle 8 and the upper nozzle 10 are slightly shifted because of the moment load due to the liquid pressure reaction force F. With respect to the lower nozzle 8, the lateral deviation (deviation within the horizontal plane) is small because of the simple cantilever structure by means of the horizontal arm 7, and the problem is not serious. On the other hand, the upper nozzle 10 are supported in a cantilever fashion to the column 3 via the multiple stages of the sliders and the vertical shaft 23, so that the lateral deviation .epsilon. is not negligible as indicated by phantom line in FIG. 2. It is considered that this is because the liquid pressure reaction force F functions to push up the upper nozzle 10 at a distance L from the axis of the vertical shaft 23, and a moment load F.times.L acts on the U slider 26 and the vertical shaft 23, and a bending deformation occurs.
The deviation due to the moment load occurs not only during the taper machining, but also ordinary machining when the upper nozzle is at the reference position and the wire is normal to the XY plane (horizontal plane), and in which the taper machining is not effected. This is because the distance L exists even at the reference position of the upper nozzle 10 although it varies during the driving of the slider mechanism. As a result, the wire guide 11 supporting the wire 16 deviates, and the error in the angle of the wire 16 with respect to the workpiece 6 is increased, and the machining precision is degraded.